Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2019
Leases [Abstract]
Operating Lease, Liability			$ 15
Finance Lease, Liability			$ 130
Operating Leases, Rent Expense	$ 7	$ 9
Long-term Purchase Commitment, Amount	149
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	131
Capital Lease Obligations, Current	2
Capital Leased Assets, Gross	57
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation	8
Purchase Commitment, Remaining Minimum Amount Committed	47
Deferred Costs, Leasing, Accumulated Amortization		3
Interest Expense, Lessee, Assets under Capital Lease		4
Operating Leases, Rent Expense, Contingent Rentals	14
Operating Leases, Rent Expense, Sublease Rentals	$ 4	$ 4